1. Technical Field
This invention relates to an air bag device for protecting a passenger from an impact developing at the time of a frontal collision of a vehicle, and more particularly to an air bag device in which an airbag member can be quickly and positively inflated and expanded, with its central portion instantaneously inflated.
2. Related Art
There are known conventional air bag devices which reduce an impact acting on the passenger by supplementing an effect or a seat belt during the impact during a frontal collision of a vehicle exceeding a predetermined value. In such an air bag device, an airbag member is inflated and expanded by high-pressure gas and the passenger is protected from the impact by a cushioning effect of the inflated and expanded airbag member. This air bag device is mounted in a central portion of a steering wheel or in an instrument panel. Usually, the airbag member used in the air bag device is formed by superposing a reverse-side cloth of a circular shape (on which an inflator for generating gas at the time of a frontal collision is mounted) and a front-side cloth of a circular shape (which abuts against the passenger upon inflation and expansion of the air bag) together and then sewing the peripheral edge portions of the two cloths together.
For example, there is disclosed an air bag device including the type of airbag member in which an outer peripheral portion of a flat portion is folded back toward its central portion to form a folded-back portion, and this folded-back portion is sewn to the flat portion (see U.S. Pat. No. 5,899,495). In the air bag device disclosed in this U.S. Pat. No. 5,899,495, when gas flows into the airbag member, that portion of the airbag member disposed inwardly of the sewn portions projects a predetermined distance toward the passenger. Then, when the internal pressure of the airbag member increases, the sewn portions are opened, so that the folded-back portion is inflated and expanded laterally. Therefore, the airbag member can be expanded widely in a flattened manner at the front side of the passenger, while suppressing the inflation of the central portion thereof. And besides, the airbag member can be obtained merely by folding the ordinary airbag member at the predetermined portions thereof and then by sewing the folded portion, and therefore the production process can be simplified, and the production cost can be reduced.
However, in the air bag device disclosed in this U.S. Pat. No. 5,899,495, although the airbag member can be expanded widely in a flattened manner at the front side of the passenger, there are occasions when the inflation of the central portion is inadequate. And besides, the air bag is so constructed as to be inflated and expanded when the sewn portions are opened, and therefore the inflation and expansion of the airbag member are influenced by an opening condition of the sewn portions thereof, and therefore the reliability is inferior.